This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to MRI data acquisition using a propeller k-space data acquisition.
A known k-space acquisition method is Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction, or Propeller. See, for example, Pipe “Motion Correction with PROPELLER MRI: Application to Head Motion and Free-Breathing Cardiac Imaging”, MRM 1999; 42 (5): 963-969. With this method, small strips (blades) of k-space are sampled after slice excitation, with the strips successively rotated by an incremental angle until the entire k-space is sampled. The overlapping circular region of the blades around the center of k-space can be used for self-navigation and calibration.
Single-shot echo-planar imaging (ssEPI) has a leading role in many MR applications, including diffusion-weighted imaging (DWI) and functional MRI (fMRI). Its high frame rate is of particular interest for fMRI and other dynamic scans, such as perfusion-weighted MRI. For nondynamic scans (e.g., DWI), the snapshot feature of EPI is an important factor in avoiding the shot-to-shot random starting phase of each readout induced by the large diffusion gradients in concert with bulk motion. However, there are a multitude of problems associated with ssEPI. Geometric distortions could be considered as the main problem of the technique, along with many other undesired effects such as Nyquist ghosting, T*2 blurring, Maxwell effects, and eddy currents (especially for DW-EPI). The magnitude of these artifacts (aside from Nyquist ghosts) in EPI is inversely scaled by the speed with which k-space is traversed along the phase-encoding (PE) direction (i.e., the pseudo bandwidth). The pseudo bandwidth is determined by two factors: 1) the time between two consecutive echoes in the EPI readout train, and 2) the phase field of view (FOV). The latter may be decreased by the use of parallel imaging, whereby the phase FOV is reduced by a factor of R during the scan and then unfolded to the nominal phase FOV during the reconstruction process. In a similar manner, per-interleave phase FOV reduction is also obtained with interleaved EPI (iEPI). However, with interleaved EPI (in particular DW iEPI and non-DW iEPI) with severe patient motion, the phase inconsistencies between interleaves may cause problematic ghosting.
An alternative fast acquisition technique that is particularly suitable for DWI and is robust in the presence of patient head motion is PROPELLER (FIG. 1a). With PROPELLER the same 2D circle in the center of k-space is sampled in each readout, and thus the technique is inherently 2D navigated and able to correct for in-plane motion. Moreover, because of the RF-refocused nature of the blades in PROPELLER, the images are free from the geometric distortions present in EPI. The drawbacks of PROPELLER are that it is slower compared to EPI and is quite specific absorption rate (SAR)-intensive, which becomes an issue primarily at higher fields.
To combine the inherent 2D navigation for motion correction with EPI's rapid acquisition, the RF-refocused readout may be replaced by an EPI readout. This was recently proposed for diffusion imaging and in combination with parallel imaging. However, the off resonance sensitivity of these propeller readouts is in fact identical to a standard ssEPI sequence, since the pseudo receiver bandwidth remains unchanged. In contrast to Cartesian EPI, off-resonance distortions in propeller EPI appear as a blurring effect instead of a simple displacement of the object in the PE direction. Given this undesired blurring, former propeller EPI did not have any advantage over ssEPI because the 2D navigation of several blades can never outperform ssEPI in terms of robustness to motion. Moreover, with ssEPI, the unidirectional geometric distortions resulting from either susceptibility gradients or eddy currents may be corrected with the use of postprocessing techniques.